1. Field of the Invention
This invention relates to a process of preparing a catalyst for use in production of methacrylic acid and the catalyst and also relates to a process of preparing methacrylic acid in the presence of the catalyst, more particularly, to the process of preparing the catalyst and the same which is suitable for preparing methacrylic acid by gas phase oxidation and/or oxidative dehydrogenation of at least one compound selected from the group consisting of methacrolein, isobutyraldehyde and isobutyric acid and the process for preparing methacrylic acid in the presence of the catalyst.
2. Description of the Prior Art
There have been many proposals concerning a catalyst for preparing methacrylic acid by gas phase oxidation of methacrolein, isobutyraldehyde and unsaturated aldehyde and a process thereof. For example, in the case of using methacrolein, there have been also many proposals about improving a catalyst such as heteropoly-acids. These proposals were divided into two categories. One is concerning a composition of a catalyst and the choice of the composition and another is concerning a regulation of catalyst properties and a process thereof with good reproducibility. According to the later, for example, it is described that the preferred specific surface area of a catalyst ranges from 0.01 to 5 m2/g and 0.01 to 50 m2/g in Japan Open-Laid No.49-116022 and Japan Open-Laid No.50-37710, respectively. In spite of these specification, this catalyst is not suitable for industrial use because of excessively high reaction temperature and low selectivity.
Many proposals concerning the control of pore structure of a catalyst have been one. For example, Japan Open-Laid No.51-136615 and Japan Open-Laid No.55-73347 propose a catalyst which is achieved by adding organic substances such as ultivalent alcohol and polyvinyl alcohol and then curing them. However, the organic substances, which is used as additive and has a high decomposition temperature, will bum by heat treatment and thereby cause sintering and reduction of a catalyst and a problem such as low reproducibility of a catalyst still remains. Japan Open-Laid No.4-367737 proposed a process, wherein an organic polymer such as polymethylmethacrylate and polystyrene, which decomposes into a monomer and vaporizes at a relatively low temperature, is added, this method is also insufficient in the reproducibility of a catalyst.
In addition, a molding process, where a dried powder containing catalytic composition with the particle size adjusted in the range of 1 to 250 xcexcm is used as a raw material, was proposed for the preparation of a catalyst in Japan Open-Laid 8-10621. However this process is not insufficient for industrial use, since this process has a problem of low reproducibility and also needs a more complicated drying process.
A catalyst for industrial use is generally used as a molded article. A molded article is produced by the process such as press molding, extrusion molding, rolling molding, Marumerizer molding, fluidized granulation, centrifuge fluidized coating process and the like. However, it is very hard to mold without lowering performance of a catalyst, and performance of the obtained catalyst is usually insufficient in reproducibility. For example, examples using press molding process were disclosed in Japan Open-Laid No.63-315148, Japan Open-Laid No.8-10621, Japan Open-Laid No.10-258233 and Japan Patent Gazette No.3-58776. According to all examples of them, a process of a press molding was not investigated in detail and these disclosures were insufficient from the view point of the reproducibility.
An object of the invention is to provide a process of preparing a catalyst for use in production of methacrylic acid and the catalyst. Another object of the invention is to provide a process for preparing methacrylic acid in the presence of the catalyst. The present inventors found that the above mentioned object can be achieved by molding a raw material including a powder containing phosphorus and molybdenum at the specific range of pressure and have accomplished the invention.
In other words, this invention provides a process for preparing a catalyst and the same for use in production of methacrylic acid by gas phase oxidation and/or oxidative dehydrogenation of at least one compound selected from the group consisting of methacrolein, isobutyraldehyde and isobutyric acid. The process comprises:
molding a raw material including a powder containing phosphorus and molybdenum at the pressure of 50 kgf/cm2 to 5000 kgf/cm2 into an article; and
calcining the molded article to obtain a catalyst
The invention also provides a process for preparing methacrylic acid characterized by gas phase oxidation and/or oxidative dehydrogenation of at least one compound selected from the group consisting of methacrolein, isobutyraldehyde and isobutyric acid with molecular oxygen or a gas containing molecular oxygen in the presence of the catalyst produced by a method of the invention.
The process for preparing the catalyst of the invention is characterized by molding a raw material including a powder containing phosphorus and molybdenum as an essential component at the pressure of 50 to 5000 kgf/cm2 into an article and calcining the molded article to obtain a catalyst.
First, the raw material used for the invention will be described. The raw material includes a powder containing phosphorus and molybdenum(hereinafter referred as P/Mo containing powder).
The P/Mo containing powder may contain other elements, for example vanadium, copper, zinc, silver, arsenic, antimony, zirconium, cerium, alkali metal, alkaline earth metal and the like besides phosphorus and molybdenum.
Preferably, a weight of the P/Mo containing powder is reducible from 0 to 40%, preferably 2 to 35%, more preferably 4 to 30% by weight with respect to the weight of the P/Mo containing powder while the P/Mo containing powder is heated at 300xc2x0 C. in the atmosphere or an inert gas. The weight of the P/Mo containing powder will be substantially constant after an elapse of 2 hours of heat treatment. The weight loss of the P/Mo containing powder can be determined at that time if a weight of the P/Mo containing powder will be substantially constant after an elapse of 30 mins. or an hour. When a weight of the P/Mo containing powder is reduced over 40% by weight with respect to the weight of the powder, the fluidity of the powder deteriorate because the powder becomes sticky.
Preferably the P/Mo containing powder substantially comprises of particles having a diameter of 150 to 1500 xcexcm, preferably 250 to 1000 xcexcm, more preferably 250 to 800 xcexcm. In the case of particle size below 150 xcexcm, the fluidity of the P/Mo containing powder and runnability of molding conducted by the following molding process deteriorate and the selectivity for COx of the resultant catalyst is liable to increase because a pore size of the molded article becomes smaller. On the other hand, in the case of particle size over 1500 xcexcm of the P/Mo containing powder, the amount of the P/Mo containing powder sucked into the mortar of a molding apparatus is variable and it causes to vary the molding pressure. As a result, mechanical properties of the resultant molded article and performance of the catalyst therefrom are liable to be lower because a pore size of the molded article becomes larger.
The P/Mo containing powder is prepared by a conventional method for a catalyst known to those of ordinary skill in the art. For example, compounds which contain phosphorus and molybdenum respectively are dissolved and dispersed into the medium usually into water to obtain a solution or slurry (hereinafter referred to as suspension).
The suspension is condensed, dried and calcined where necessary to prepare a powder. The suspension can be prepared by a conventional method such as evaporating concentration process, oxide mixing process and coprecipitation process and the like known to those skilled in the art, as long as the composition of the suspension is not remarkably uneven. Specifically, the suspension containing phosphorus and molybdenum can be prepared from the compounds such as oxide, nitrate, carbonate, hydride, chloride and ammonium salt of each element. For example, the following compounds can be used for the preparation of the suspension. Examples of compounds having molybdenum include molybdenum trioxide, molybdic acid and ammonium paramolybdate and the like. Examples of compounds having phosphorus include orthophosphoric acid, phosphorus pentoxide and ammonium phosphate and the like. Examples of compounds having vanadium include vanadium pentoxide, ammonium metavanadate and the like. Examples of compounds having copper, zinc, silver, arsenic, antimony, zirconium and cerium and the like include oxide, nitrate, carbonate, hydride, chloride, ammonium salt of each element. Examples of compounds having alkali metal and alkaline earth metal include nitrate, carbonate, hydride, chloride. Molybdophosphoric acid, molybdovanadophosphoric acid and ammonium salt of these acids and alkali metal salt also can be used.
The suspension can be dried by a well known methods such as evaporation drying, spray drying, drum drying, flash drying, drying under a reduced pressure, freeze drying and the like. Spray drying, drum drying and flash drying are preferable among them because these methods can simplify a process of pulverizing and screening particles with the desired diameter, preferably between 150 to 1500 xcexcm.
In addition to the P/Mo containing powder described above, the raw material used for the invention further comprises a lubricant and/or a reinforcement. Examples of the lubricant include graphite, talc, starch, palmitic acid, stearic acid and stearate. Examples of the reinforcement include inorganic fiber such as glass fiber and any type of whisker which are known for improving the mechanical properties of a catalyst and an attrition resistance. A lubricant and/or a reinforcement can be mixed with the P/Mo containing powder to obtain the raw material.
The raw material of the invention is molded at the pressure of 50 to 5000 kgf/cm2, preferably 200 to 3500 kgf/cm2, more preferably 600 to 2500 kgf/cm2 into an molded article. In the case of molding below the pressure of 50 kgf/cm2, mechanical properties of the resultant molded article falls and performance of the catalyst therefrom is getting lower because a pore size of the catalyst becomes large. In the case of molding over the pressure of 5000 kgf/cm2, the selectivity for COx of the resultant catalyst is liable to increase because a pore size thereof becomes too small. An excellent performance and a good reproducibility of the catalyst can be achieved by molding at the range of the pressure of 50 to 5000 kgf/cm2.
The raw material can be molded into a molded article by any molding apparatus. The molded article has no limitation on its shape. Examples of the shape of the molded article include pellet, sphere, ring and tablet. The preferred shape of the molded article is a ring, because the shape of a ring expands surface area of a catalyst and thereby enables oxidation more effectively. The molded article also has no limitation on its size and usually has the size of 1 to 20 mm, preferably 3 to 5 mm in the average diameter.
The molded article is calcined to obtain a catalyst at a temperature preferably between 300xc2x0 C. to 600xc2x0 C., more preferably 350xc2x0 C. to 500xc2x0 C. The preparation of the catalyst may be carried out in the presence of a nitrogen containing heterocyclic compound such as pyridine and quinoline. In this case, N containing heterocyclic compound may be removed by heat treatment after molding.
According to the process of preparing the catalyst of the invention, the catalyst with excellent performance can be produced with good reproducibility. The catalyst prepared by the process of the invention has no limitation on its composition, as long as the catalyst contain phosphorus and molybdenum and can be used for preparing methacrylic acid by oxidation and/or oxidative dehydrogenation of at least one compound selected from the group consisting of methacrolein, isobutyraldehyde and isobutyric acid. The preferable composition of the catalyst may be shown by the following equation(1).
PaMobVcAdBeCfOxxe2x80x83xe2x80x83(1)
wherein P is phosphorus, Mo is molybdenum, V is vanadium, A is at least one element selected from the group consisting of copper, zinc and silver, B is at least one element selected from the group consisting of arsenic, antimony, zirconium and cerium, C is at least one element selected from the group consisting of alkali metal and alkaline earth metal, O is oxygen and a, b, c, d, e, f and x denote the ratio of atom of P, Mo, V, A, B, C and O, respectively and a=0.5 to 4, b=12, 0 less than cxe2x89xa64, 0 less than dxe2x89xa63, 0xe2x89xa6exe2x89xa65, f=0.01 to 4 and x is the number determined by the valance states of the other elements.
According to the invention, the process for preparing methacrylic acid by gas phase oxidation and/or oxidative dehydrogenation of at least one compound selected from the group consisting of methacrolein, isobutyraldehyde and isobutyric acid with molecular oxygen or a gas containing molecular oxygen is characterized by the oxidation and oxidative dehydrogenation being carried out in the presence of the catalyst which is produced by the process of the invention.
In the invention, the term xe2x80x9cpreparing methacrylic acid by oxidation and/or oxidative dehydrogenation of at least one compound selected from the group consisting of methacrolein, isobutyraldehyde and isobutyric acidxe2x80x9d denotes the general term of preparing methacrylic acid by oxidation of methacrolein, preparing methacrylic acid by oxidation and oxidative dehydrogenation of isobutyraldehyde, preparing methacrylic acid by oxidative dehydrogenation of isobutyric acid and the combination of these processes.
The gas phase oxidation and oxidative dehydrogenation for preparing methacrylic acid of the invention is not restricted in terms of the condition and an apparatus except for using the catalyst of the invention and can be carried out by a conventional method known to the skilled in the art.
A raw material supplied to oxidation and oxidative dehydrogenation(hereinafter referred as a raw material for methacrylic acid) is preferably a mixed gas including at least one compound selected from the group consisting of methacrolein, isobutyraldehyde and isobutyric acid, molecular oxygen and an inert gas as diluent. The mixed gas preferably contains 1 to 10 volume % of at least one compound selected from the group consisting of methacrolein, isobutyraldehyde and isobutyric acid. The mixed gas contains molecular oxygen and the volume of molecular oxygen is usually 1 to 10 times as much as that of at least one compound selected from the group consisting of methacrolein, isobutyraldehyde and isobutyric acid. The mixed gas also contains an inert gas as diluent and examples of an inert gas include nitrogen, carbon dioxide and steam and the like. Especially, steam is preferable because steam inhibits generation of by-product and is suitable for improving yield of methacrylic acid. Gas phase oxidation is carried out by reacting the mixed gas at 200 to 400xc2x0 C. under normal pressure to 10 atm. at the space velocity of 100 to 5000 hrxe2x88x921(STP) in the presence of the catalyst of the invention. When methacrolein is used as a raw material for methacrylic acid, methacrolein is not necessarily pure. A gas containing methacrolein which is obtained by contact catalytic reaction of isobutylene, t-butanol and methyl-t-butylether can be used. Especially this gas can be preferably used for the industrial process of preparing methacrylic acid.
According to the process for preparing methacrylic acid of the invention, ethacrylic acid can be produced in high yield by gas phase oxidation and/or oxidative ehydrogenation of at least one compound selected from the group consisting of methacrolein, isobutyraldehyde and isobutyric acid.